Wave transmission and shaping



Feb; 17, 1942. R. A. HEISING I 3 9 WAVE TRANSMISSION AND SHAPING FiledOct. 7, 1938 TIME L TIME mylsipu 5 DIVISION mm, IDZDLZ Mun/FLEXTRANSMITTER, R, R2 4 a RECEIVE)? FIGS 'INVENTOR Eb By RAHE/S/NG AT QRNEPatented Feb, 17, 1942 um'rEof sTArEs PATENT oF IcE WAVE TRANSMISSIONAND'SHAPING Raymond A. Heislng, Summit, N. -J.,-assignor to BellTelephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application October 7, 1938,Serial No. 233,761

11 Claims.

waves and more particularly to signal wave shaping. In one broad aspectthe invention relates to selective damping or curbing. In a relatedaspect the invention relates to suppression of the trailing portions ofsignal wave impulses in circuits having highly reactive impedance.

The invention has particular application to V time-division multiplextransmission and will be illustrated in connection with such a system.In time-division multiplex the different signal input circuits are ineffect connected to the line or other transmitting medium one at a timein rapid succession such that fragmentaryportions of the differentsignals are sent over the system in rotation. If the system in itsentirety possessed only resistive impedance (so that there were nothingto limit the frequency band transmitted) the different signal fragmentswould theoretically retain their original shape and there/would be nooverlapping of the successive fragments. According to the viewpoint ofthe Fourier analysis, an infinite band of frequencies would betransmitted for each signal fragment and the wavecomponents'representing this infinite band would cancel out at all timesexcept withinthe interval devoted to the particular signal fragment, andwithin that interval they would add up vectorially to produce the exactshape of the pulse. Any attempt to restrict the transmitted band to lessthan infinite width would result in distorting the signal impulse. Afilteror resonant circuit or long transmission line having aconsiderable reactive component of impedance, for example, would distortthe impulse and cause overlapping of successive impulses and result incross-talk.

Since pure resistance systems are never realized in practice, butselective circuits are a practical necessity particularly in highfrequency modulated wave transmission practice, the problem ofoverlapping impulses caused by the trailing portion of the impulse wavebecomes of basic importance in time-division multiplex systems.

. The general object of the invention is to provide variable damping ofa circuit or system with time in such a manner as to correct fordistortion.

In .the specific form of the invention to be illustrated and describedherein a vacuum tube has its plate impedance connected acros a resonantcircuit (or other circuit to be damped) and this plate impedance is kepthigh during the times when the desired portion of a wave is passing butis made low during the time when undesired wave energy would otherwisepersist in the circuit. The plate impedance of such tube is controlledby its grid and the control may be derived from the signal wave impulsesor separately exercised.

The nature of the invention and its various objects and features will bemore fully understood from the following detailed description of theparticular systems chosen for illustration.

Fig. 1 is a diagrammatic sketch of a radio system to which the inventionis applicable;

Fig. 2 is a similar sketch of a transmission line to which the inventionis applicable;

Figs. 3 and 9 are schematic circuit diagrams of forms which theinvention may take and which may be incorporated into the systems ofFigs. 1 and 2 in a manner to be described; and

Figs. 4 to 8 show wave diagrams to be used in the further description ofthe invention.

In Fig. 1, incoming lines Z1, la, in at the left or transmitting stationlead up to the rotary distributor i0 and each has one side connected 'toa respective distributor segment, the other side of each line beingconnected to one input terminal of radio transmitter Hi. The rotatingarm of the distributor I0 is connected to the other input terminal oftransmitter l2.

The lines Z1, Z2, etc., may be telephone, telegraph or other signalinglines. Each linehas a filter F connected in it to limit the frequencyband to the appropriate range for the type of signal being transmitted,which in the case of speech might extend from 200 to 3000 cycles by wayof example.

An entirely analogous arrangement is employed at the right-hand orreceiving terminal yvhere in this case the lines 1'1, 1'2, Zn arereceiving lines associated by way of rotary distributor ID with radioreceiver i2.

One intermediate radio repeater 20, typical of a succession of spacedrepeaters intermediate between the terminal stations, is shown forreceiving radio waves from I2, reamplifying them and sending them out,usually on a difierent wave-length, for reception at [2.

The arms of distributors l0, l0 are operated by motors l1, l1 drivenfrom oscillators I8, l8 or other sources in any known manner to rotatein close synchronism and phase with each other and with a frequency ofrotation at least twice as high as the highest frequency passed by thefilters F, F, that is, at least twice as high as the highest utilizedsignal frequency. Fragments of signals on lines Z1, Z2, Zn are thus sentinto radio transmitter I! in rapid succession and eventually the samefragments are recovered at terminal l2 and distributed tothecorresponding receiving lines so that in each receiving line thefragments piece together to re -form the signal intended for that line.

Radio transmitter I 2 may be assinned to incor- Yporate the necessaryapparatus for-"producing and sending out the-signabmodulated .radiowaves, such=asoscillation generating, modulating:

and amplifying devices ofknown together tor-further on. receiver 'i2"may be asv sumed to incorporate the typestof'frequencyselecting'circuits, amplifying and demodulating devices. Theintermediate repeater or repeaters incorporate elements of a receiver,and a transmitter togetherwith' selective cir- 1 'cuits for selectivelyreceiving-the waves, amplifying them and. retransmitting' them withshifted frequency. 'The manner in which the invention is to be practicedin connection with aradio' system such as that just described will bevgiven in detail presently. It will be understood that the description oftheradio' system has been given in V quite general terms because theinvention is of.

broad application and,- therefore, not dependent tuned circuits or otherreactive impedance elements are used. -Actuall ycross-talk will occur.in some degree'between every'channeland every other channelin-asy'stem' other-'thanithe'ideal.

in which an infinitely wideband of frewith frequency selective circuitsto be referred quencies is used and all impedances are purely resistive.

-' The invention provides a novel means'and method ofreducing'cross-talk in a system in pass through the system.

upon any particular type of system or the use of specific types ofdevices. p

, Fig. 2 will now be described briefly as another general type oftransmission system with which the invention may be practiced. Thisdiffers from the Fig. 1 system mainly'in' type of transmission mediumused, the medium being line L, suchas an open-wire line, a coaxialconductor line or a pair in a lead sheathedcable. Theterminal stationsmay be the same as inFig. 1, the line conductors of line L replacing theantennae and ground conductors of Fig. 1.

It will be noted that the distributors III, II!

' have active segments alternating with idle or blank segments, thelatter being unconnected to any of the lines so that there are timeintervals elapsing between the timesin which connections are being madewith the successive signal lines. This is illustrated in Fig. 4. Thetime interval I is allotted to line 11 ,(time extending to the right inthis figure) but this interval is divided into parts a and a. Therotating arm is in contact with the line 11 active segment during a,resulting in an impulse during time a, and is passing over the nextsuccessive idle segment during a. Similarly for lines 12, etc., impulsesb and c are indicated as being transmitted followed by spaces b and c'.In the figure the active and idle periods are indicated as equal, whichis preferable, although the invention is in no wise Y limited to the useof any particular arbitrary ratio.

The Figs. 4 to 8 are placed in vertical alignment so that time periodscorresponding to the active or impulse producing periods (a, b, c) andthe b period or further. It is this trailing wave One manner ofaccomplishing this will"now be described in connection with Fig. 3..This figure represents a jtuned amplifier of special .con-

struction according to the invention and is to be considered asincorporated into the systems of Figs. 1 'and'2- at every point wheretuned circuits are used. For example, in Fig. 1, the trans- .mitter itincludes 'a circuit or circuits'tuned tothe radio wave beingtransmitted; likewise the repeaters 26 each contain a plurality of tunedcircuits and so may the receiver 12'. The same is true of the terminalsand repeaters of Fig. 2.

Fig. 3 is shown asrhaving an initial untimed stage 39 followed by twotuned stages 3|, 32, the number not being significant. The interstageplate impedance for the plate of stage 30 has suitable dampingresistance 33 to prevent this impedance from having a resonantcharacteristic arranged to have their plate impedances varied to varythe damping of tuned circuits '34 and 35. For this purpose oscillatorcircuit 38 is provided. It is tuned to the switching or commutatingfrequency of the rotary distributors and may have its frequencycontrolled by any suitable means to maintain its frequency in closesynchronism and phase therewith. At the transmitting and receivingstations oscillator 38 may be the oscillator i8 or l8 which rotates thechannel switching distributor or an oscillator which governs thefrequency of the distributor. At repeater points it would be a separateoscillator. In the circuit of Fig. 3 an amplifier tube 39 has its inputconnected across the output of stage 39. Its circuit is less broadlytuned thanthat of amplifier 3B for it is not necessary to preventoverlapping of the channel signals in order to derive the channelfrequency ripple. The circuit 42 may therefore be more sharply tunedthan is necessary forseparation of channel signals. Tube 43 is adetector which secures the' ripple frequency current and impresses it onthe grid of the-oscillator 38 holding the oscillator in step with theswitching frequency at all times.

The wave produced by oscillator 38 is of the form shown in'Fig. '7, asine wave of rather large implitude. This is impressed simultaneously onthe grid circuits of shunt tubes 38 and 31,

Tuned circuit- 'into a long trailing wave.

The operation of the circuit of Fig. 3 is as follows: A periodicsuccession of wave trains, ideally like those of Fig. 5 but in practicemore like those of Fig. 8, is impressed on the input stage and isamplified by stages 36, 3| and 32. The tuning in the circuit, at 34 and35, and other reactive eifects tend to cause the wave to trail out intosome such form as that in Fig. 6 as already explained. During the activeor used periods (1,1), 0, etc., the wave from oscillator 38 is executingits negative swings (see Fig. 7) and is driving the grids of shuntingtubes 36 and 31 negative. The grids of these tubes are normally suitablybiased to a high-impedance condition, for example to near cut-oif orbeyond by batteries 45 and 46 so that these tubes are maintained intheir nontransmitting, high or infinite impedance condition during thea,b, 0, etc., periods. Consequently, the tuned circuits 34 and 35 areeffectively unshunted. During the idle intervals a, b, c',

however, tubes 36 and 31 have their grids driven positive by oscillator38 (of Fig. 7) and the plate impedances drop to a low value, effectivelyshunting the tuned circuits 34 and 35. This action effectively shuntsoff the trailing portions of the waves, giving a resulting wave shapesimilar to that of Fig. 8 with reduced cross-talk. Another way ofviewing the action is to consider that the tendency of the circuits toproduce the trailing wave efiect in the idle periods is reduced byremoving their reactive impedance efiects and rendering theircharacteristics more nearly resistive. Resistances 4i and 4| may be usedto supplement the plate resistances of tubes 36 and 31 if necessary.

Fig. 2 differs from Fig. l in that the medium of Fig. 1 (the ether) hasno reactive component whereas a transmission line does have reactivecharacteristics that tend to prolong an impulse This effect in the caseof any uniform line is proportional to the length. By using sumcientlyshort repeater sections (1. e. a suihcient number of repeaters in anygiven case) this efiect can be kept below a specified maximum on eachsection. By employing a periodically damped repeater, such as in Fig. 3,as described, the portion of trailing wavealready existing in the idleintervals in the arrival wave as well as that which the'tunedcircuits ofthe repeater tend to produce can be removed as already described. Insuch case the tubes 36, 37 damp not only the tuned circuits 34, 35 butalso the preceding line section that tends to continue discharging intothe repeater. Where the line section between two repeaters is short, thetrailing wave produced by it will not be sufficient to require wipingout of itself but it willbe wiped out by tube 36 when this tubeeliminates the trailing wave of circuit 34. Where theline betweenrepeaters is long enough to produce trailing waves that in themselveswill give rise to cross-talk, a damping tube may be suitably coupledacross the line at a point ahead of stage 30 and its impedancecontrolled by oscillator 38 in the same manner as in'the case of tubes36 and 31. The type of coupling employed, whether direct or through atransformer, should be such as to allow the tube to offer a minimumshunting effect across the line for the signal proper while highlyattenuating the trailing wave.

Fig. 9 shows an alternative type of self-damping repeater that may beused in the systems of Figs. 1 and 2 in the same way as the Fig. 3 type.In Fig. 9 the oscillator has been omitted and the wave form of thesignal impulses is relied on to control the damping.

The stages 50, 5| and 52 may be the same as stages 30, 3| and 32 of Fig.3, and tuned circuits 54, 55 correspond to tuned circuits 34 and 35. Thedamping tubes are 56 and. 51, having their plate impedances connected inshunt to tuned circuits 54, 55. A rectifier tube 66 is connected inseries with inductive winding 6| and stopping condenser 62 across theplate-cathode terminals of stage 52 and by rectifying a part of theoutput current, produces impulses of the switching or commutatingfrequency in inductance 6i. Condenser 63 is a radio frequency by-passcondenser and coil 64 is a direct current or low frequency choke coil.Coil 65 is inductively coupled to coil 6| and is connected in the gridcircuits of tubes 56 and 51.

In operation, when the wave trains are impressed on the circuit there isa tendency due to the time constants for the wave to increase inamplitude, reach a maximum and decrease in some such manner as in Fig.6. During the portion of growing amplitude rectifier 6B is passing adirect current pulse from the growing high frequency amplitude acrosscircuit 55, which pulse induces an electromotvie force in winding 65 ofsuch direction as to send the potentials of the grids of tubes 56 and 5lnegative, causing their plate impedances to be high. During the portionof the wave trains that have decreasing amplitude, the potential inwinding 65 reverses, causing tubes 56 and 5! to offer low shuntingimpedance and attenuate the radio frequency trailing wave.

Figs. 3 and 9 illustrate two absorber tubes only, but for best resultsevery tuned circuit or coupled circuit in a repeater, transmitter orreceiver should have an absorber tube. Two methods of control have beenillustrated by way of example, and it will be obvious that these may bemodified variously to suit conditions.

The rotary distributors illustrated in Fig. l are of the mechanicaltype,'for simplicity. In practice it may be perferable to use some non-.mechanical type such as a cathode ray type or gas or vacuum tubescyclically operating, or other suitable type. An example of the formeris disclosed in P. Mertz application Serial No. 192,471, filed February25, 1938, and an example of the latter is disclosed in U. S. Patent toHolden No. 2,099,065, issued November 16, 1937. In any case, synchronousoperation may be obtained by suitable means known in the art and thesame type of synchronizing means can be used at each station where tunedcircuits occur to synchronously insert resistances during theinter-impulse spaces to dampen the circuits.

The input and output connections to the amplifier may be made inconventional fashion in most cases. The input circuit may be the sourceof low power time-division carrier signals at a transmitting station, ormay be ordinary antenna and tuned circuits reasonably broadly tuned atrepeater or receiver locations. The output circuit may work onto a line,onto an antenna or may work into a power stage similarly controlled tothose indicated, or it may go to the detector.

The signals emanating from an amplifier of this type will be asindicated in Fig. 8 and are not identical with the hypothetical ones ofFigs. 4 and 5. These signals, however, will be proportional in amplitudeto the original signals and will, therefore, carry the originalinformation. In succeeding amplifier stages there should be of the waveform of incoming signals of the form in Fig. 8 as possible.

In an amplifier of this type the periodic introduction of resistance inthe tuned circuits produces a type of broadening action. The broadeningof the circuits, however, is not identical with that of circuits havingfixed resistance. In this case the circuits will receive thosefrequencies for which they are tuned and have suitable constants. Theintroduction of the resistance to damp out the trailing wave produces inthese circuits additional frequencies for which the circuit does notrespond readily before the introduction of the resistance. The band ofdelivered frequencies is broader than the band of receiveu frequencies.

Since the invention is regarded as of basic scope, it is not to beconstrued as limited in its broader aspects to the means chosen forillustrative embodiment herein, but its scope is indicated in the claimswhich follows.

What is claimed is:

1. In combination, means to impress on a cir cuit electrical impulses inperiodically recurring time intervals with intervening spacingintervals, said circuit having reactance tending to prolong the impulsesinto the subsequent time intervals, a damping resistance the value ofwhich can be varied between wide limits connected in sh'unt relation tosaid reactance, and means to suppress the prolonged parts of saidimpulses in said circuit comprising means operating to vary the value ofsaid resistance between its limits in time-correspondence with saidimpulses to provide low shunting resistance across said reactance insaid spacing intervals.

2. The combination according to claim 1 in which said means comprises areactive impedance and rectifier for distinguishing the rising portionsfrom the falling portions of the impulses, and means controlled by saidrectifier in response to the falling portions of the impulses forvarying said resistance to its low value.

3. The combination according to claim 1 in which said means comprises agenerator of wave impulses of the periodicity of said electrical im-'pulses, and means controlling the value of said resistance by thegenerated impulses.

4. In a wave repeater circuit, amplifier tubes and tuned circuitsconnected to said tubes for selective amplification of waves ofparticular. frequency, and means operating -in synchronism with acomponent frequency of the waves being 6. In a wave repeater for highfrequency waves having a relatively low frequency component, a resonantcircuit tuned to the high frequency of the wave being repeated,'a spacedischarge tube having its space path shunted across said resonantcircuit, and a control circuit operating at said relatively lowfrequency for varying the space impedance of said tube periodically atsaid low frequency to dampen the resonance of said circuit periodicallyat said relatively low frequency.

. 7. A repeater according to claim 6, in which said control circuitcomprises a rectifying and filtering circuit for deriving a voltage ofsaid low frequency from the low frequency component of said highfrequency waves.

8. In combination, means generating electrical impulses with idleperiods'between successive impulses, a system for transmitting suchimpulses having a portion tending to trail out one impulse to overlap intime another impulse, and means for suppressing the trailing waveportions comprising means operating only in said idle periods for highlyattenuating said trailing wave portions, said last means comprising aspace discharge tube having its space path impedance shunted across theportion of the system tending to trail out said fragments, a grid forsuch tube, and means for driving the potential of said grid in thepositive direction during each idle period.

9. A wave shaping circuit for a transmission circuit having highlyreactive impedance comprising a space discharge device having its spacepath impedance bridged across said highly reactive impedance, a grid forcontrolling said space amplified for periodically varying the damping ofsaid tuned circuits.

5. In combination, a circuit having a highly reactive impedance, a spacedischarge device having its plate impedance shunted across saidimpedance, means transmitting waves through said circuit having animpulse frequency, said highly reactive impedance tending to distort theshape of the impulses and means to counteract such distorting actioncomprising means periodically reducing the plate impedance of saiddevice to a low shunting value at the impulse frequency.

path impedance, and means to impress varying potentials on said gridsuch as to maintain its space path impedance a maximum during thebuilding 'up t me of impulses traversing said transmission circuit andminimum in the dyingdown time of said impulses.

10, In a signaling system for signals having a wave frequency and anenvelope frequency, said system having a highly reactive impedancetending to cause trailing out of the waves, a space discharge tube forperiodically damping said impedance to reduce such trailing outefiects;said tube having its space discharge path connected across saidimpedance, and means for controlling the impedance of said tube at-theenvelope frequency.

11. In a system for transmitting waves in periodically spaced wavetrains with nominally idle periods between wave trains, a circuit madeselective to thc wave frequency by use of suitable reactances, means toattenuate waves tending to persist in the system in the nominally idleperiods comprising a variable resistance connected across said selectivecircuit, and means to. control the value of said resistance at the wavetrain frequency to cause the resistance to damp the selective circuitduring the nominally idle periods.

RAYMOND A. HEISING,

